The invention relates to a process for the preparation of alkyl-substituted anilines by the reaction of alkyl-substituted phenols in the vapor phase with ammonia in the presence of an aluminum oxide catalyst.
It is known that phenols can be reacted in the vapor phase with ammonia, under pressure and over catalysts containing aluminum oxide, to give the corresponding anilines (U.S. Pat. No. 1,935,209; U.S. Pat. No. 2,013,873; U.S. Pat. No. 3,272,865, German Auslegeschrift No. 2,026,053 and German Offenlegungsschrift No. 2,003,842). Although a large number of phenols are listed in the stated publications as being suitable starting compounds for the reaction, only a few phenols, in particular phenol itself, m- and p-cresol and 3,5-xylenol, are employed in the examples given therein (U.S. Pat. Nos. 3,272,865; 1,935,209 and 2,013,873).
According to German Offenlegungsschrift No. 2,516,316, difficulties arise in amination reactions with substituted phenols. Thus, for example, it is mentioned that up to 10% of by-products are formed when m-cresol is used. To avoid these disadvantages, German Offenlegungsschrift No. 2,516,316 recommends the addition of toluene. According to the examples of German Offenlegungsschrift No. 2,516,316, 2 parts by volume of toluene are added per part by volume of phenol, and the toluene must subsequently be separated off from the product mixture by distillation, condensed, and recycled. In addition to the increased outlay of apparatus required for this purpose, the use of the readily inflammable toluene at the high reaction temperatures is also unacceptable for reasons of safety.
According to Japanese Patent Application No. 49-29 176 (1974), m-toluidine is formed with a selectivity of only 81% in the reaction of m-cresol with ammonia in the gas phase.